1. Field of the Invention
The invention pertains to medical ultrasound imaging systems which utilize an array of transducer elements to transmit ultrasound energy. In particular, it pertains to a pulse generator circuit which enables an array of transducer elements to transmit a focused beam of coherent ultrasound energy and to control the amplitude of the pulses transmitted from each transducer element.
2. Description of the Prior Art
A typical ultrasound imaging system used for medical imaging includes at least one ultrasound transducer, usually in the form of an array of transducer elements. The transducer is usually connected to transmitter stage circuitry, receiver stage circuitry and whatever devices may be necessary to effect mechanical scanning of the transducer. The transmitter stage comprises a generator of electrical excitation signals which are sent to the transducer. The transducer converts these signals into periodic pulse trains of ultrasound mechanical energy.
Transducer arrays used in medical imaging systems come in many forms, such as linear arrays, phased arrays, annular arrays and collimated image arrays. A transducer array usually consists of a plurality of transducer elements disposed on a surface in some designed arrangement such as a row, matrix or other geometric pattern. The individual elements or groups of elements are actuated or pulsed in sequence to transmit a beam of ultrasound energy at a target. Ultrasound echoes are returned from the target and may be received on the same transducer elements. The pulse echo data received is then interpreted and displayed to produce an image.
In the technology of medical imaging with ultrasound, the principles of the linear array and the phased array are well known, though the technical terminology can be semantically misleading. For the purposes of this application, the following definitions will be used. A linear array is an electronically scanned array of similarly sized and shaped elements arranged in an extended line, side by side. A group of contiguous transducer elements are electronically selected from the extended array, are pulsed for transmission and then sometimes used for reception of resultant echoes. The selected group of transducer elements is then commutated one or more positions along the array and the process repeated to scan successive parallel regions in the body. A linear array projects beams of ultrasound energy perpendicular to the face of the transducer element. The image format is usually rectangular.
A phased array refers to a short linear array of transducer elements, the transmitted energy being deflected from the normal perpendicular beam by inserting delays in the pulse signal to each element. Similarly the received pulse echo is steered in angle by inserting delays in the signal path from each angle before summation. The resulting image is pie-shaped and accomplishes the so-called sector scan.
Electronic focusing of both types of arrays is possible by a different set of time delays. By introducing time delays, focusing or phasing is possible to improve lateral resolution over a particular depth range inside a target. Focusing improves lateral resolution (beam width) in the focal zone and it improves sensitivity because of higher energies produced in the focal area.
The performance of transducer arrays has been improved by the use of controlled delay elements which produce electronic focusing in the reception of pulse echoes. In these systems, each transducer element receives pulse echo signals from many points in the target. Signals which are received by the individual elements are appropriately delayed and summed. In general, focusing over a large focal zone is accomplished quite easily in the receive mode since the echo arrival time to the center element can be used to prescribe the corresponding set of time delays of other elements in the system. Dynamic focusing may also be utilized for reception of pulse echo signals.
Most modern pulse echo ultrasound imaging systems utilize a linear transducer array and employ dynamic electronic focusing during signal reception. Most linear array transducer systems also employ a fixed focus during pulse transmission. The axial resolution of such an array depends on the length of the pulse. Resolution in the transverse plane depends on the elevational dimension of the array. The focusing is usually achieved using a fixed mechanical lens.
The prior art has made some attempts to introduce horizontal (longitudinal) focusing at the transmitter stage of an ultrasound imaging system. In the transmit mode time delays have been used to steer beams by constructive and destructive interference of all pulsed signals. Whether in the transmit mode or the receive mode, these prior art time delays are frequently implemented in analog circuits, which trigger an oscillator, or by the analog delay of an existing pulse train. However, the use of analog delays introduces jitter in transmission, cross-talk and other forms of time noise.